Acoustic printing is a potentially important direct marking technology. It still is in an early stage of development, but the available evidence indicates that it is likely to compare favorably with conventional ink jet systems for printing either on plain paper or on specialized recording media, while providing significant advantages of its own.
Drop on demand and continuous stream ink jet printing systems have experienced reliability problems because of their reliance upon nozzles with small ink ejection orifices which easily clog. Acoustic printing obviates the need for such nozzles, so it not only has greater intrinsic reliability than ordinary ink jet printing system, but also is compatible with a wider variety of inks, including inks which have relatively high viscosities and inks which contain pigments and other particulate components. Furthermore, it has been found that acoustic printing provides relatively precise positioning of the individual printed picture elements ("pixels"), while permitting the size of those pixels to be adjusted during operation, either by controlling the size of the individual droplets of ink that are ejected or by regulating the number of ink droplets that are used to form the individual pixels of the printed image. See a copending and commonly assigned United States patent application of Elrod et al, which was filed Dec. 19, 1986 under Ser. No. 944,286 on "Variable Spot Size Acoustic Printing".
When an acoustic beam impinges on a free surface (i. e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which the beam exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. These principles have been applied to prior ink jet and acoustic printing proposals. For example, K. A. Krause, "Focusing ink Jet Head," IBM Technical Disclosure Bulletin, Vol 16, No. 4, September 1973, pp. 1168-1170 described an ink jet in which an acoustic beam emanating from a concave surface and confined by a conical aperture was used to propel ink droplets out through a small ejection orifice. Lovelady et at. U.S. Pat. No. 4,308,547, which issued Dec. 29, 1981 on a "Liquid Droplet Emitter," showed that the small ejection orifice of the conventional ink jet is unnecessary. To that end, they provided spherical piezoelectric shells as transducers for supplying focused acoustic beams to eject droplets of ink from the free surface of a pool of ink. They also proposed acoustic horns driven by planar transducers to eject droplets of ink from an ink coated belt.
Spherical piezoelectric transducers are suitable for use in low and moderate resolution acoustic printers. Such a transducer can be designed so that the acoustic beam it generates comes to an essentially unaberrated focus at or near the free surface of a pool of ink, thereby minimizing the variables that need to be controlled to achieve stable operation. Unfortunately, however, the mechanical strength of known piezoelectric materials imposes a design constraint on the minimum permissible thickness of a shell-like transducer, with the result that the upper end of the useful frequency range for these transducers is somewhere in the vicinity of 25 MHz. In a liquid, such as water, the wavelength of a 25 MHz acoustic beam is approximately 60 microns, so the upper limit on the printing resolution that can be achieved, using an ink having an acoustic velocity comparable to that of water, is only about 200 spots per inch. Furthermore, these shells are usually several milimeters in diameter.
To increase the resolution which can be achieved and to provide a less cumbersome and lower cost technique for manufacturing arrays of relatively stable acoustic droplet ejectors, a copending and commonly assigned United States patent application of Elrod et al, which was filed Dec. 19, 1986 under Ser. No. 944,698 on "Acoustic Lens Arrays for Ink Printing" is introducing acoustic lenses for performing the beam focusing function. That application is hereby incorporated by reference. However, the acoustic lens is not limited to use in arrays. Indeed, it has been found that the acoustic lens is extremely well suited to all forms of acoustic printing because its aperture need not be much larger than the wavelength of the acoustic wave in the solid which defines the lens.